"Mozilla" formerly was the codename of the Netscape Navigator and is currently used by the most browsers. The version 5 of the Netscape Navigator was the last, unreleased version, before the switch to the successor Mozilla Firefox. You will find the Mozilla/5.0 token on the most modern browser.

Windows NT 5.1

This is the platform token, used by Microsoft Windows XP. The end-of-live date of Microsoft Windows XP was the 08. Apr. 2014.

Release date: Fri., 24. Aug. 2001

AppleWebKit/537.36

AppleWebKit/537.36 is used since Chrome/27.0.1412.0

Release date: Thu., 14. Feb. 2013

KHTML

KHTML is a free HTML layout engine developed by the KDE project. This engine is used by the Konqueror web browser. Webkit is based on KHTML. KHTML is written in C++ and released under LGPL.

like Gecko

It´s not a Geckeo browser, but behaves like a Gecko Browser. Gecko is the open source browser engine designed to support open Internet standards and is uses in several browsers like Firefox, SeaMonkey and other.